This invention relates to apparatus for cutting a metal workpiece by melting the workpiece with a melting tool such as a plasma-arc torch. Such a torch operates by ionizing a column of gas with an electric arc to produce temperatures up to 30,000 degrees F. A high velocity, high temperature plasma stream and electric arc strike the workpiece with the heat serving to melt the metal while the high velocity stream blows away the molten metal.
Apparatus which utilizes a plasma-arc torch is disclosed in Brolund et al U.S. Pat. No. 4,063,059. In the apparatus disclosed in that patent, a duct is disposed beneath the workpiece in alinement with the torch. Molten metal falls downwardly into the duct while fumes are sucked into the duct and away from the work area by an exhaust system. Prior to the fumes being discharged from the exhaust system, foreign particles are filtered from the fumes in order to provide a relatively clean discharge to atmosphere.
The apparatus disclosed in the Brolund et al patent functions adequately under certain circumstances. When the torch, however, is operated substantially continuously over a long period of time, the heavy and prolonged flow of the plasma stream and the molten metal into the duct causes the duct to get so hot that holes may be burned in the duct and welds may be broken loose. In addition, the molten metal tends to build up within the duct as slag and tends to clog the duct. In some cases, the fumes sucked out of the duct are extremely hot and contain so many particles that the filter becomes clogged in a relatively short period of time.